The present disclosure relates to the packaging and testing of spatial light modulators.
In manufacturing MEMS devices, such as spatial light modulators, multiple spatial light modulators (SLMs) are commonly fabricated on a semiconductor wafer. The individual (SLMs are then respectively sealed in chambers and subsequently separated into individual encapsulated dies. The chambers typically include transparent windows through which the SLMs receive and output optical signals. The operations of the SLMs are preferably tested at the wafer level before they are individually packaged and used in final product apparatus.
In the wafer level testing of normal, encapsulated integrated circuits, such as microprocessors, the test pads of each device are readily accessible to test probes. Often debris is formed by the probes making mechanical, vibrating contact with the test pads, but such debris has no deleterious impact on the integrated circuit device that is already encapsulated and protected.
However, in the case of testing MEMS devices, such as SLMs, where mechanical contact with test pads before the encapsulation is common, the resulting debris will remain inside MEMS cavity and often can wreak havoc with the devices, such as by causing shorts across the delicate metal traces or mechanically blocking the moving parts of the MEMS.
It is highly desirable to test the electro-optical devices at the wafer level before their singulation into individual dies and before their attachment onto the final substrates. Although it would be possible to test the devices after singulation, such testing requires alignment of each individual die before test electrical contact can be made to it. Such die level testing therefore requires expensive handling and alignment equipment and long alignment times, increasing cost.
In addition, certain electro-optical devices, such as SLMs, cannot be tested until all devices on the wafer have been respectively enclosed in individual sealed chambers. This means that testing is done after wafer level encapsulation and sealing.
It is possible to encapsulate and seal the individual devices on the wafer and then expose the electrical contacts of the devices by removing portions of the encapsulation material from the areas covering the electrical contacts. Such a process is difficult, as sections of the encapsulation have to be removed without damaging the electrical contacts or the electrical interconnections of the underlying devices. Moreover, after dicing the wafer into individual dies, it is difficult to clean the dicing debris and to obtain reliable electrical contacts so that testing can be carried out with a high degree of confidence.
In the case of SLMs, mechanical and optical testing is required in addition to electrical testing. Before testing, some SLMs having moving mirrors that make mechanical contact with a landing pad require an anti-stiction material to be applied within the chamber. For such material to be effective, the chamber must be sealed and the anti-stiction material activated. If this material is not applied before testing, mirrors may stick during the test, making the test results unreliable and making further testing impossible.